This invention relates to ultrasound probes including a hydrophilic couplant.
The testing and examination of complex structures, without damaging them, is now of immense importance in a wide range of industrial and medical situations. The use of ultrasound is perhaps the most ubiquitous such methodology. It is now used both to detect and characterise static defects or anomalies in metal or fibre composite structures, and to image biological systems in real time. A large range of techniques, from simple manual scanning to computer-controlled multi-axis tomography systems, are in use or under development.
The testing process involves generating an ultrasound beam from a source of ultrasound (normally a piezoelectric crystal) and introducing the sonic energy into the specimen to be examined. A detector is then arranged to receive the energy that is reflected, transmitted or scattered, depending upon the operating system and the objectives of the examination. The resolution of the process depends in part upon the frequency of the ultrasound used; high frequencies are needed to resolve small defects, and thus the frequency range now used in ultrasonic examination is typically 1-50 MHz.
In all cases, there is a need to introduce the interrogating beam into the specimen and remove the resulting sonic signal after passage through the specimen through the surface of the specimen. In most cases, it is convenient for the specimen to be surrounded by air, but the interface, i.e. samplexe2x86x92airxe2x86x92transducer, represents a discontinuity at which sonic energy is lost by mismatch, scattering or reflection, thus seriously reducing the effectiveness of the test process. In addition, the effect of a given air gap varies with the frequency of the applied signal, becoming more significant as the frequency increases.
To overcome the mismatch at the air interface between the transducers (generation and detection) and the specimen, liquid and dry couplants have been applied. Thus, in one case, the air gap is eliminated by filling it with a liquid, of which the most common is water. For automatic scanning, the jet probe has been developed. In this device, a water jet is directed from the generating transducer onto the target, and the ultrasound passes through the continuous water layer to the specimen. This works well, provided that the specimen is resistant to water or can be completely dried after examination. This method is the most widely used technique in the aerospace industry, for producing through C-scans of large components; its limitations include: the need to maintain laminar flow in the water jet, which limits the geometries that can be examined; the difficulty in properly removing all the water from a complex structure, which is especially important in those structures that are susceptible to corrosion, the requirement for pumping equipment; and the difficulty in avoiding air bubbles that may influence results undesirably.
A number of high viscosity aqueous gels have been developed to allow water to be applied in a form which will reduce run-off or pooling, but such materials do not accommodate large surface deformation nor do they prevent the water from drying up quickly and disturbing the examination. They also require careful cleaning of the specimen, if their removal is necessary. They exhibit high attenuation and/or can be used only at low transmission frequencies.
As an alternative to liquid couplants, a range of soft elastic (polymeric) materials has been developed, conventionally based upon natural or artificial rubbers which are applied as xe2x80x9cpadsxe2x80x9d to make contact between the transducer and the specimen, or in the form of tires when applied to a wheel probe. This class of device is pressed onto the surface of the specimen and tracked over the surface to build up a picture of the subsurface over a significant area; one application is the detection of cracks in railway track, when the intention is to examine some miles of track at any one time. However, considerable pressure is often required to ensure a good contact, and this is not always possible. Further, existing materials do not have the necessary ultrasound properties to make a good match between the transducer and the range of specimen materials available. They exhibit high attenuation and/or can be used only at low transmission frequencies.
Desirably, a couplant material should be safe and readily manufactured into a range of coupling systems. In addition, it should satisfy the following requirements:
(i) show low attenuation to ultrasound;
(ii) be transparent to a wide range of acoustic frequencies;
(iii) posses an acoustic impedance similar to that of common specimen materials, so as to reduce energy reflection or scattering at the interface (and ideally offer the possibility of controlling the impedance to match other systems); and
(iv) possess a structure which allows the elastic properties to be varied widely (to make it possible to couple to surfaces of differing mechanical properties and shapes).
Cross-linked hydrophilic materials are characterized by the ability to take in large amounts of water, or other polar liquids, and reach a state of stable hydration without suffering solution or long-term degradation. Such materials are now readily available having equilibrium water contents in the range 10 to 98% by wet weight (i.e. materials which absorb up to 15 times their dry weight). Until recently, they found application only in ophthalmic optics. However, their mechanical stability, and the ability to control their hydraulic properties (water uptake and permeability), and their gas permeability, when hydrated, have led to a number of other applications in the fields of bio-implants, wound care and drug release where the safety of the materials has been established over a long period of time.
U.S. Pat. No. 3,921,442 discloses an ultrasound probe including a hydrophilic polymer couplant. The only polymer that is disclosed is polyHEMA, i.e. a homopolymer of 2-hydroxyethyl methacrylate. This has a maximum water content of about 38%, and insufficient strength for any dynamic application. PolyHEMA is not capable of transmitting a frequency above 5 MHz, without high attenuation of the transmitted signal. It is noteworthy that there is no disclosure of any specific device or use of the couplant, in U.S. Pat. No. 3,921,442.
It is clear that no single material or combination of materials now in use fulfills the requirements for ultrasound couplants, given above. In particular, their acoustic impedance is generally low.
The present invention is based on the discovery that, although U.S. Pat. No. 3,921,442 discloses a system unsuitable for commercial use, certain cross-linked hydrophilic copolymers are suitable and effective for use as ultrasonic couplants, notably in fault detection systems. According to the present invention, an ultrasound probe includes, in addition to conventional components such as a source of ultrasound or means of transmitting ultrasound from a remote source, a cross-linked hydrophilic material as an integral couplant; the hydrophilic material is capable of transmitting a frequency in the range of 5 to 20 MHz, and exhibits attenuation of the transmission of less than 1.5, and preferably less than 1, dB.mmxe2x88x921 at 5 MHz. It is not essential that the material is capable of transmitting at all frequencies in the given range, although that may be preferred; it may also be capable of transmission at frequencies above 20 MHz. Nor is it essential that the probe is used to transmit frequencies in the given range.
A probe according to the present invention can be simple and portable. Its use avoids damage to structures caused by the action of free water. There is no requirement for water pumping equipment or water tanks. The test piece can be of any size. It may also be non-regular in shape or have a rough surface.
The hydrophilic material can be supplied as in integral part of the probe, or independently, and in a range of acoustic impedances allowing close matching to that of the test structure. Further, hydrophilic materials can combine the advantages of dry and liquid couplants, without their disadvantages. Thus, for example, by contrast to gels, a hydrophilic material can be combined with an ultrasound probe to provide an integral device, e.g. by using the shrink-fitting properties of hydrophilic materials. Further, unlike gels, the cross-linked nature of such hydrophilic materials prevents them from flowing. Therefore, they are spatially defined, and as such, combinations of hydrophilic materials can be used, e.g. with the aim of focusing the ultrasound waves. The invention allows the use of polymers whose properties can be varied controllably within a given couplant device.